This invention relates to the art of power supplies and, more particularly, to a controller for a series resonant power converter of the switching regulator type.
Power converters are known in the art and typically serve to accept energy from an unregulated energy source, such as a voltage source, and derive therefrom a regulated voltage which is applied to a load circuit. The regulation function is performed by interposing a regulating device between the source of energy and the load circuit. It is known in the prior art to utilize a regulating device, such as a controlled variable impedance interposed between the source and the load. In such case, the variable impedance is continuously varied in its impedance magnitude in order to maintain a constant voltage or current at the load circuit. Such variable impedances dissipate significant amounts of the power transmitted from the source to the load.
Another form of regulating device known in the prior art includes switching type regulating devices interposed between the source of energy and the load. These operate in a discontinuous manner in controlling the rate of energy transmission and, hence, consume less power during the regulating operation than do the variable impedance type regulating devices. The switching device has two modes of operation, fully on and fully off. The switching device is periodically turned on for a time interval to permit energy transfer for purposes of maintaining the power output at a predetermined level.
Typically, such switching type regulating devices employed in power converters utilize semiconductor devices, such as power transistors, as the switching devices. These devices are turned fully on, or saturated, or fully off during operation. When fully on, the semiconductor devices are conducting and little or no power is dissipated. Also, when nonconducting or fully off no power is dissipated therein. Power is, however, dissipated in such a semiconductor device during the time interval of switching from a nonconducting condition to a conducting condition and vice versa. It is during the switching time interval that a substantial amount of power may be dissipated in such a semiconductor device, and if large enough this may severely damage the semiconductor device.
Power converters employing switching type regulators utilizing semiconductor devices as power switchers have typically been operated such that the power switches supply a squarewave signal to a power transformer which couples the power converter to a load. The squarewave operation requires that the power switches dissipate energy whenever voltage and current are interrupted. Consequently, there is a switching loss which is directly related to the operating frequency of the power switches. This has contributed to limiting such power switches to low power applications during high frequency operation. Otherwise, they are operated at a low frequency using, larger components and more space.
It is desirable to provide power converters which are cost effective and which occupy a small amount of space. This, then, necessitates high frequency operation, such as in excess of 20 KHz and preferably at substantially higher frequencies, such as 250 KHz, while transferring large amounts of power, such as on the order of 200 watts or more. In order to achieve such high frequency operation of power switches, it has been proposed to employ sinusoidal operation as opposed to the squarewave operation typically employed in the prior art. Such a proposal is found in the article "High Frequency Power Conversion With FET-Controlled Resonant Charge Transfer", by R. H. Baker, published in the PCI April, 1983 proceedings, pages 130-133. This articles proposes an operation wherein sinusoidal current pulses flow through a series resonant circuit including the primary winding of a power transformer by way of a power switch. Each sinusoidal current pulse terminates to a zero level before the voltage forcing function is withdrawn. As a consequence, the power switches turn on and off at zero current to thereby provide lower component switching loss to reduce component stress.
As reported in Baker, supra, alternate sinusoidal current pulses flow through a series resonant tank circuit. During one half cycle a current pulse flows in a first direction through the primary winding by way of a first power switch and a first capacitor. During the next half cycle a current pulse flows in the opposite direction through the transformer primary winding by way of a second power switch and a second capacitor. These two power switches are each turned on for a fixed time duration with the on times being separated by a minimum fixed interval or dead time during which one switch is turned off and the other is turned on. It is assumed in Baker, supra, that the current flowing through the resonant tank circuit is zero during this dead time interval and, hence, that switching operation of the power switches will not dissipate energy. This, however, ignores fluctuations in the unregulated power supply or in the loading of the output. These flunctuations may vary the time duration of current flow. Thus, a short on the output may cause the trailing edge of the current pulse to lag somewhat into the dead time interval. In such case the switching on and off of the power switches will cause energy to be dissipated, resulting in inefficiency of power transfer and a high potential of component damage or destruction.
Additionally, Baker's dead time interval has a minimum duration of two microseconds at the maximum frequency of operation under full power conditions wherein the full cycle of operation is 12 microseconds in duration. Each switch is on for four microseconds. Consequently at full power, current flows only for 8 microseconds out of a 12 microsecond cycle, or for only 2/3 of a cycle. This limits the amount of power that may be transferred during an operating cycle at full power conditions.